The Boston Marathon: How Do High Temperatures Affect Runners?

What happens when athletes run in hot weather?The 116th Boston Marathon took place on Monday, and the day's higher-than-normal temperatures wreaked havoc with the runners. With the mercury creeping up to and over 80 degrees by mid-day, 4,300 or less-experienced runners opted to skip the race this year, and most the 22,426 who opted to run the 26.2-mile course saw slower times than usual.

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The heat claimed a notable victim early on: The defending men's champion, Geoffrey Mutai of Kenya -- who won last year's Boston Marathon with the fastest time in world history -- was forced to drop out at the 18-mile mark because of cramping, The Associated press reported.

Even seasoned athletes are at risk for cramping, heat exhaustion, and heatstroke when they compete on warm days. Amby Burfoot, who won the Boston Marathon in 1968, participated in a 2009 experiment at the University of Connecticut, during which he completed two hourlong runs, one at a cool 53 degrees and the other at a sweltering 90 degrees. In an article for Runner's World, he described the effect that the higher temperatures had on his body:

"During the hot run, my heart rate soared to 175, about 96 percent of my max. My temperature spiked to 103.5, close to the edge of heatstroke, which can potentially occur when your core temperature reaches 104.0. My lactic acid climbed above 4.0, the point most physiologists define as the lactate threshold where the leg muscles no longer function efficiently. And my plasma volume contracted by more than 10 percent, which, coupled with a 2.6 percent total dehydration, forced my heart to work harder to push blood to my legs. All this at a pace I considered comfortable. If I had run much longer or harder at 90 degrees, it's possible that I could have staggered into heat illness, the precursor to the heatstroke hurt zone."

Though temperatures in the 80s are high and uncomfortable, marathon racers in Boston have seen worse. In 2002, one runner died on the course. A couple of generations ago, during the 1909 race known as "The Inferno," the day reached 97 degrees; in 1976's "Run for the Horses" runners started in 100-degree heat and those who made it to the finish line were hosed down with water.

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